Citric acid is used as a food acidulant, and in pharmaceutical, industrial and detergent formulations. The increased popularity of liquid detergents formulated with citric acid has been primarily responsible for growth of worldwide production of citric acid to about 700 million pounds per year which is expected to continue in the future.
Citric acid is produced by a submerged culture fermentation process which employs molasses as feed and the microorganism, Aspergillus Niger. The fermentation product will contain carbohydrates, amino acids, proteins and salts as well as citric acid, which must be separated from the fermentation broth.
There are two technologies currently employed for the separation of citric acid from fermentation broths containing the same. The first involves calcium salt precipitation of citric acid. The resulting calcium citrate is acidified with sulfuric acid. In the second process, citric acid is extracted from the fermentation broth with a mixture of trilauryl-amine, n-octanol and a C.sub.10 or C.sub.11 isoparaffin. Citric acid is reextracted from the solvent phase into water with the addition of heat. Both techniques, however, are complex, expensive and they generate a substantial amount of waste for disposal.
The patent literature has suggested a possible third method for separating citric acid from the fermentation broth, which involves membrane filtration to remove raw materials or high molecular weight impurities and then adsorption of contaminants onto a nonionic resin based on polystyrene or polyacrylic resins and collection of the citric acid in the rejected phase or raffinate and crystallization of the citric acid after concentrating the solution, or by precipitating the citric acid as the calcium salts then acidifying with H.sub.2 SO.sub.4, separating the CaSO.sub.4 and contacting cation- and anion-exchangers. This method, disclosed in European Published Application No. 151,470, Aug. 14, 1985, is also a rather complex and lengthy method for separating the citric acid. In contrast, our method makes it possible to separate the citric acid in a single step and recover the citric acid in a much simplified process. Succinctly stated, the citric acid is adsorbed selectively by the adsorbent and purified citric acid is desorbed by a desorbent, for example, water or a dilute acid, e.g., sulfuric acid, hydrochloric acid, nitric acid or phosphoric acid.
The establishment of pH below the pK value in an adsorbent separation of citric acid from other acids is disclosed in U.S. Pat. No. 2,664,441, but there is no suggestion of or rationale for the application thereof to the instant separation of citric acid from mixtures thereof with non-acidic components.